A carbon nanotube can be visualized as a sheet of hexagonal graph paper rolled up into a seamless tube and joined. Each line on the graph paper represents a carbon-carbon bond, and each intersection point represents a carbon atom.
In general, carbon nanotubes are elongated tubular bodies, which are typically only a few atoms in circumference. The carbon nanotubes are hollow and have a linear fullerene structure. The length of the carbon nanotubes potentially may be millions of times greater than their molecular-sized diameter. Both single-walled carbon nanotubes (SWNTs), as well as multi-walled carbon nanotubes (MWNTs) have been recognized see “Nanotubes from Carbon” by P. M. Ajayan, Chem. Rev. 1999, 99, 1787-1799, the disclosure of which is hereby incorporated herein by reference).
Carbon nanotubes are currently being proposed for a number of applications since they possess a very desirable and unique combination of physical properties relating to, for example, strength and weight. Carbon nanotubes have also demonstrated electrical conductivity. See Yakobson, B. I., et al., American Scientist, 85, (1997), 324-337; and Dresselhaus, M. S., et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, pp. 902-905. For example, carbon nanotubes conduct heat and electricity better than copper or gold and have 100 times the tensile strength of steel, with only a sixth of the weight of steel. Carbon nanotubes may be produced having extraordinarily small size. For example, carbon nanotubes are being produced that are approximately the size of a DNA double helix (or approximately 1/50,000th the width of a human hair).
Considering the excellent properties of carbon nanotubes, they are well suited for a variety of uses, from the building of computer circuits to the manufacturing of heat-reflective material, and even to the delivery of medicine. As a result of their properties, carbon nanotubes may be useful in microelectronic device applications, for example, which often demand high thermal conductivity, small dimensions, and light weight. Perhaps most promising is their potential to act as nano-wires and even tiny transistors in ultradense integrated circuits. One potential application of carbon nanotubes that has been recognized is their use in flat-panel displays that use electron field-emission technology (as carbon nanotubes generally make excellent pipes for the high-energy electrons). Further potential applications that have been recognized include electromagnetic shielding, such as for cellular telephones and laptop computers, radar absorption for stealth aircraft, nano-electronics (including memories in new generations of computers), and use as high-strength, lightweight composites. Further, carbon nanotubes are potential candidates in the areas of electrochemical energy storage systems (e.g., lithium ion batteries) and gas storage systems.
Various techniques for producing carbon nanotubes have been developed. As examples, methods of forming carbon nanotubes are described in U.S. Pat. Nos. 5,753,088 and 5,482,601, the disclosures of which are hereby incorporated herein by reference. The three most common techniques for producing carbon nanotubes are: 1) laser vaporization technique, 2) electric arc technique, and 3) gas phase technique (e.g., HIPCO™ process), which are discussed further below.
In general, the “laser vaporization” technique utilizes a pulsed laser to vaporize graphite in producing the carbon nanotubes. The laser vaporization technique is further described by A. G. Rinzler et al. in Appl. Phys. A, 1998, 67, 29, the disclosure of which is hereby incorporated herein by reference. Generally, the laser vaporization technique produces carbon nanotubes that have a diameter of approximately 1.1 to 1.3 nanometers (nm). Such laser vaporization technique is generally a very low yield process, which requires a relatively long period of time to produce small quantities of carbon nanotubes. For instance, one hour of laser vaporization processing typically results in approximately 100 milligrams of carbon nanotubes.
Another technique for producing carbon nanotubes is the “electric arc” technique in which carbon nanotubes are synthesized utilizing an electric arc discharge. As an example, single-walled nanotubes (SWNTs) may be synthesized by an electric arc discharge under helium atmosphere with the graphite anode filled with a mixture of metallic catalysts and graphite powder (Ni:Y;C, as described more fully by C. Journet et al. in Nature (London), 388 (1997), 756. Typically, such SWNTs are produced as close-packed bundles (or “ropes”) with such bundles having diameters ranging from 5 to 20 nm. Generally, the SWNTs are well-aligned in a two-dimensional periodic triangular lattice bonded by van der Waals interactions. The electric arc technique of producing carbon nanotubes is further described by C. Journet and P. Bernier in Appl. Phys. A, 67, 1, the disclosure of which is hereby incorporated herein by reference. Utilizing such an electric arc technique, the average carbon nanotube diameter is typically approximately 1.3 to 1.5 nm and the triangular lattice parameter is approximately 1.7 nm. As with the laser vaporization technique, the electric arc production technique is generally a very low yield process that requires a relatively long period of time to produce small quantities of carbon nanotubes. For instance, one hour of electric arc processing typically results in approximately 100 milligrams of carbon nanotubes.
Thus, both the laser vaporization technique and electric arc technique can only produce small quantities of SWNTs, See A. G. Rinzler et al, Appl. Phys. A, 1998, 67, 29-37; C. Journey and P. Bernier, Appl. Phys. A, 1998, 67, 1-9. More recently, Richard Smalley and his colleagues at Rice University have discovered another process, the “gas phase” technique, which produces much greater quantities of carbon nanotubes than the laser vaporization and electric arc production techniques. The gas phase technique, which is referred to as the HIPCO process, produces carbon nanotubes utilizing a gas phase catalytic reaction. The HIPCO process uses basic industrial gas (carbon monoxide) under temperature and pressure conditions common in modern industrial plants to create relatively high quantities of high-purity carbon nanotubes that are essentially free of by-products. The HIPCO process is described in further detail by P. Nikolaev et al. in Chem. Phys. Lett., 1999, 313, 91, the disclosure of which is hereby incorporated herein by reference.
While daily quantities of carbon nanotubes produced using the above-described laser vaporization and electric arc techniques are approximately 1 gram per day, the HIPCO process may enable daily product of carbon nanotubes in quantities of a pound or more. Generally, the HIPCO technique produces carbon nanotubes that have relatively much smaller diameters than are typically produced in the laser vaporization or electric arc techniques. For instance, the nanotubes produced by the HIPCO technique generally have diameters of approximately 0.7 to 0.8 nanometer (nm).
Carbon nanotubes are commonly produced (e.g., using the above-described techniques) in relatively long, highly tangled ropes. For example, SWNTs produced by the HIPCO process (which are available from Carbon Nanotechnologies,. Inc.) generally comprise relatively long (e.g., >4 micrometers (μm)) and relatively thick (e.g., 20-100 nm) ropes formed by a plurality of highly tangled carbon nanotubes.
A desire often exists for a nanotube structure that is shorter than the relatively long tubes commonly produced. Shortened single-walled carbon nanotubes (e.g., SWNTs having length ≦1 μm) will have a rich chemistry due to their higher chemical processability. For example, shortened SWNTs can be further sorted by length, chemically functionalized, solubilized and chromatographically purified (J. Liu et al, Science 1998, 11, 834-840; S. Niyogi et al, J. Am. Chem. Soc. 2001, 123, 733-734). Accordingly, such shortened SWNTs are the fitting subjects of a new branch of organic chemistry, a molecular nanotechnology of great promise. Processable shortened SWNTs can be further polymerized and copolymerized to form nanotube-based polymer composites and copolymers which will find applications in the areas of electromagnetic shielding coatings for military aircraft and ships as well as mobile telephones (e.g., cellular telephones) and laptops, antistatic coatings (e.g., for automobiles), and organic thin film devices for micro-electronics and micro-optoelectronics. Shortened SWNT materials, due to their high density of open ends, will also find applications in electrochemical energy storage systems for lithium batteries and hydrogen storage systems for fuel cells. In addition, shortened SWNTs, after appropriate conversion (solubilization, exfoliation and chemical functionalization), are promising one-dimensional building blocks for constructing advanced nanoscale structures which may find important applications in molecular electronics.
Various techniques have been proposed for shortening (or “cutting”) carbon nanotubes to result in nanotubes having a length shorter than that at which the nanotubes are produced. One technique for shortening the length of carbon nanotubes utilizes prolonged sonication of carbon nanotubes to a mixture of concentrated sulfuric and nitric acids, as described more fully in “Fullerene Pipes” by Jie Liu et al. published in Science, volume 280 (pages 1253-1256) on May 22, 1998, the disclosure of which is hereby incorporated herein by reference. Liu et al. found that the length distribution of the carbon nanotubes exposed to the concentrated sulfuric and nitric acids shortened systematically with exposure time to the acid.
One disadvantage of Liu et al's method is that the yield of shortened SWNTs is low. The total yield (after shortening and polishing steps) is approximately 30% or less, thus only a small amount of usable shortened carbon nanotubes can be obtained from the purified SWNTs using such acid-cutting technique. Also, while this acid-cutting technique does effectively shorten the length of carbon nanotubes having relatively large diameters (e.g. the SWNTs produced by laser vaporization or electric arc production techniques), it is not a suitable solution for shortening SWNTs having smaller diameters, such as those typically produced by the HIPCO process (e.g., approximately 0.7 to 0.8 nm diameter). Because of the higher chemical reactivity due to strain in small diameter nanotubes, it appears that the acid-cutting technique may not be suitable for cutting such small diameter SWNTs. For example, after 12 hour sonication of 10 mg of SWNTs (produced by the HIPCO process) in 12 ml of 3:1 mixture of concentrated sulfuric acid (H2SO4)/nitric acid (HNO3), it was found by transmission electron microscopy (TEM) that most of such small-diameter SWNTs were severely damaged.
Another proposed technique for shortening the length of carbon nanotubes utilizes diamond particles as an abrasive material for cutting such carbon nanotubes, as described more fully in “Nano-mechanical cutting and opening of single wall carbon nanotubes” by 1. Stepanek et al. in Chem. Phys. Lett., 2000, 331, 125-131, the disclosure of which is hereby incorporated herein by reference. In such technique, diamond particles are used to effectively grind the carbon nanotubes in order to cut them into shortened nanotubes. This technique has a better yield than the above-described acid-cutting technique. However, it would be very difficult to separate the shortened SWNTs from the small diamond particles, because both materials are insoluble in organic or inorganic solvents.